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Cathodic Protection
1. IEEE PES
Cathodic Protection Use
On Tank Bottoms
& Underground Piping
In Power Generation Plants
PG&E Office
San Francisco
January 18, 2007
Craig K. Meier
Corrosion Control Incorporated
2. IEEE PES
• Corrosion process for tanks and piping
• Principles of cathodic protection
• Effects of coatings
• Cathodic protection design requirements
• Selecting the proper system
• Construction implementation issues
• Compliance testing
• Continued monitoring and maintenance
15. IEEE PES
Corrosion Losses
• Will occur in all plants
• Rate of loss dependent on soil and materials used
• Coatings alone do not stop soil corrosion
• Care during construction can significantly reduce losses
21. IEEE PES
Galvanic Cathodic Protection
• Galvanic anodes have a limited current output, 10 to 30
milliamperes per anode.
• Due to the limited current output, the use of galvanic
anodes is limited to coated and electrically isolated
underground piping and underground tanks.
28. IEEE PES
Impressed Current
• Anodes produce high amounts of current, 1000 to 5000
milliamperes per anode.
• High current output allows protection of bare steel
structures.
• Structures do not necessarily need to be electrically
isolated.
29. IEEE PES
New Power Plant Design
• Involve corrosion engineer from the start
• Obtain site soil resistivitiy data
• Select piping materials
• Select coating types and quality control procedures
• Decide which structures require cathodic protection
30. IEEE PES
Cathodic Protection Design - Piping
• Soil resistivity data
• Clearly identify piping to receive cathodic protection
• Yard plan showing piping layout, isometric drawings are
difficult to use
• Location of all risers
• Type of coating
• Responsibility of isolation gaskets
• Desired service life
31. IEEE PES
Cathodic Protection Design - Tanks
• Site soil resistivity data
• Tank diameters and layout
• Equipment access
• Type of foundation
• Containment
• Monitoring capability
• Desired service life
32. IEEE PES
Design Options
• Design documents issued in construction plans
• Contract responsible for cathodic design
33. IEEE PES
Design Submittal
• Qualifications of person responsible for design,
supervision during installation and testing
– Can not mail order
• Detailed calculations for each tank and pipeline showing
current required, number of anodes, circuit resistance
and life
• Material list providing descriptions, models and
quantities
• Product data sheets
• Detailed installation plans – not diagrams
34. IEEE PES
Implementation
• The cathodic design submittals must be approved before
construction of structures
• A site meeting must be held to discuss how the cathodic
protection system will be installed, and special concerns
during construction
• Mechanical and electrical foremen should be involved in
cathodic discussions
35. IEEE PES
Avoid Costly Mistakes
• Grounding cables must be at least 12 inches from
underground pipes
• Temporary pipe supports must be removed
• Piping must be inspected for coating quality and
separation from grounding before burial
56. IEEE PES
Completed Before Testing
• All pipe flanges connected and isolated
• Pipes back filled and cathodic protection is complete
• AC power to rectifier unit
• Storage tanks contain product
• Schedule for testing and training issued
57. IEEE PES
Compliance Testing - Specialist
• Inspect cathodic systems to ensure they are properly
installed
• Test each dielectric flange for isolation
• Obtain native potentials on pipes and tanks
• Connect galvanic anodes at test stations and/or energize
rectifier(s)
• Cathodic protection systems on isolated coated structures
should operate at least 4 hours before testing
• Cathodic protection systems on bare steel or grounded
structures must operate between 12 hours and 7 days
before testing
62. IEEE PES
Emeasured = Ptank + I R
• If the current is momentarily interrupted, I=O, then;
Emeasured = Ptank + (O) (R)
• “Instant Off” potential
63. IEEE PES
Cathodic Protection Criteria
• Galvanic anodes
– -850mV or more negative with anodes connected
• Impressed current
– -850mV or more negative “instant-off”
-or-
– 100mV polarization shift
65. IEEE PES
Compliance Report
• Tabulated Data
• Narrative analysis of the data
• Compliance statement
• Calculated anode life
• Test procedures and instrumentation
• As-built drawings
• O&M Manual
66. IEEE PES
O&M Manual
• Test Procedures
• Instrument requirements
• Qualification requirements
• Frequency
• Criteria
• Trouble shooting
• Spare parts
• Wiring diagrams
67. IEEE PES
Conclusions
• Cathodic protection is viable and necessary to preserve
power plant structures
• Proper planning and coordination are essential through
out the design and construction phases, to ensure
cathodic protection systems work properly
• Properly implemented and monitored cathodic protection
will extend the service life of storage tanks and yard
piping more than 30 years
68. IEEE PES
Owner Monitoring & Maintenance
• Record rectifier output levels monthly and compare
output to target values
• Repair inoperable rectifiers within 30 days
• Inspect test stations and dielectric flanges every six (6)
months for damage and removal
• Instruct maintenance staff to replace and test removed
dielectric gaskets
• Annually, obtain potential measurements and ensure
effective cathodic protection levels
